Coesite
Crossed-polars image of coesite grain (gray) ~1 mm across in eclogite. Small, colored inclusion is pyroxene. Polycrystalline rim is quartz.
General
CategoryTectosilicate, quartz group
Formula
(repeating unit)		SiO2
IMA symbolCoe[1]
Strunz classification4.DA.35
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H–M symbol)
Space groupC2/c
Unit cella = 7.143
b = 12.383
c = 7.143 [Å]
β = 120.00°
Z = 16
Identification
Formula mass60.0843 g/mol
ColorColorless
Crystal habitInclusions in UHP metamorphic minerals up to 3 mm in size
FractureConchoidal
TenacityBrittle
Mohs scale hardness7.5
LusterVitreous
StreakWhite
DiaphaneityTransparent
Density2.92 (calculated)
Optical propertiesBiaxial
Refractive indexnx = 1.594
ny = 1.595
nz = 1.599
Birefringence+0.006
2V angle60–70
References[2]

Coesite (/ˈkoʊsaɪt/)[3] is a form (polymorph) of silicon dioxide (SiO2) that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953.[4][5]

Occurrences

In 1960, a natural occurrence of coesite was reported by Edward C. T. Chao,[6] in collaboration with Eugene Shoemaker, from Barringer Crater, in Arizona, US, which was evidence that the crater must have been formed by an impact. After this report, the presence of coesite in unmetamorphosed rocks was taken as evidence of a meteorite impact event or of an atomic bomb explosion. It was not expected that coesite would survive in high pressure metamorphic rocks.

In metamorphic rocks, coesite was initially described in eclogite xenoliths from the mantle of the Earth that were carried up by ascending magmas; kimberlite is the most common host of such xenoliths.[7] In metamorphic rocks, coesite is now recognized as one of the best mineral indicators of metamorphism at very high pressures (UHP, or ultrahigh-pressure metamorphism).[8] Such UHP metamorphic rocks record subduction or continental collisions in which crustal rocks are carried to depths of 70 km (43 mi) or more. Coesite is formed at pressures above about 2.5 GPa (25 kbar) and temperature above about 700 °C. This corresponds to a depth of about 70 km in the Earth. It can be preserved as mineral inclusions in other phases because as it partially reverts to quartz, the quartz rim exerts pressure on the core of the grain, preserving the metastable grain as tectonic forces uplift and expose these rock at the surface. As a result, the grains have a characteristic texture of a polycrystalline quartz rim (see infobox figure).

Coesite has been identified in UHP metamorphic rocks around the world, including the western Alps of Italy at Dora Maira,[8] the Ore Mountains of Germany,[9] the Lanterman Range of Antarctica,[10] in the Kokchetav Massif of Kazakhstan,[11] in the Western Gneiss region of Norway,[12] the Dabie-Shan Range in Eastern China,[13][14] the Himalayas of Eastern Pakistan,[15] and in the Appalachian Mountains of Vermont.[16][17]

Crystal structure

Atomic structure of coesite

Coesite is a tectosilicate with each silicon atom surrounded by four oxygen atoms in a tetrahedron. Each oxygen atom is then bonded to two Si atoms to form a framework. There are two crystallographically distinct Si atoms and five different oxygen positions in the unit cell. Although the unit cell is close to being hexagonal in shape ("a" and "c" are nearly equal and β nearly 120°), it is inherently monoclinic and cannot be hexagonal. The crystal structure of coesite is similar to that of feldspar and consists of four silicon dioxide tetrahedra arranged in Si4O8 and Si8O16 rings. The rings are further arranged into chains. This structure is metastable within the stability field of quartz: coesite will eventually decay back into quartz with a consequent volume increase, although the metamorphic reaction is very slow at the low temperatures of the Earth's surface. The crystal symmetry is monoclinic C2/c, No.15, Pearson symbol mS48.[18]

See also

References

  1. ↑ Warr, L. N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ↑ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). "Coesite" (PDF). Handbook of Mineralogy. Vol. 2 (Silica, Silicates). Chantilly, Virginia: Mineralogical Society of America. ISBN 0962209716. Retrieved December 5, 2011.
  3. ↑ "coesite". Dictionary.com Unabridged (Online). n.d.
  4. ↑ Coes, Jr., L. (31 July 1953). "A New Dense Crystalline Silica". Science. 118 (3057): 131–132. Bibcode:1953Sci...118..131C. doi:10.1126/science.118.3057.131. PMID 17835139. The word coesite is pronounced as "Coze-ite", after chemist Loring Coes, Jr.{{cite journal}}: CS1 maint: postscript (link)
  5. ↑ Hazen, Robert M. (22 July 1999). The Diamond Makers. Cambridge University Press. p. 91. ISBN 978-0-521-65474-6. Retrieved 2012-06-06.
  6. ↑ Chao, E. C. T.; Shoemaker, E. M.; Madsen, B. M. (1960). "First Natural Occurrence of Coesite". Science. 132 (3421): 220–2. Bibcode:1960Sci...132..220C. doi:10.1126/science.132.3421.220. PMID 17748937. S2CID 45197811.
  7. ↑ Smyth, Joseph R.; Hatton, C.J. (1977). "A coesite-sanidine grospydite from the Roberts Victor kimberlite". Earth and Planetary Science Letters. 34 (2): 284. Bibcode:1977E&PSL..34..284S. doi:10.1016/0012-821X(77)90012-7.
  8. 1 2 Chopin, Christian (1984). "Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences". Contributions to Mineralogy and Petrology. 86 (2): 107–118. Bibcode:1984CoMP...86..107C. doi:10.1007/BF00381838. S2CID 128818052.
  9. ↑ Massonne, H.-J. (2001). "First find of coesite in the ultrahigh-pressure metamorphic area of the central Erzgebirge, Germany". European Journal of Mineralogy. 13 (3): 565–570. Bibcode:2001EJMin..13..565M. doi:10.1127/0935-1221/2001/0013-0565.
  10. ↑ Ghiribelli, B.; Frezzotti, M.L. & Palmeri, R. (2002). "Coesite in eclogites of the Lanterman Range (Antarctica): Evidence from textural and Raman studies". European Journal of Mineralogy. 14 (2): 355–360. Bibcode:2002EJMin..14..355G. doi:10.1127/0935-1221/2002/0014-0355.
  11. ↑ Korsakov, A.V.; Shatskiy, V. S. & Sobolev N.V. (1998). "Первая находка коэсита в эклогитах Кокчетавского массива (First occurrence of coesite in eclogites from the Kokchetav Massif)". Doklady Earth Sciences. 359: 77–81.
  12. ↑ Smith, D.C. (1984). "Coesite in clinopyroxene in the Caledonides and its implications for geodynamics". Nature. 310 (5979): 641–644. Bibcode:1984Natur.310..641S. doi:10.1038/310641a0. S2CID 4330257.
  13. ↑ Schertl, H.-P.; Okay, A.I. (1994). "A coesite inclusion in dolomite in Dabie Shan, China: petrological and rheological significance". Eur. J. Mineral. 6 (6): 995–1000. Bibcode:1994EJMin...6..995S. doi:10.1127/ejm/6/6/0995.
  14. ↑ Wang, Xiaomin; Liou, J. G.; Mao, H. K. (1989-12-01). "Coesite-bearing eclogite from the Dabie Mountains in central China". Geology. 17 (12): 1085–1088. Bibcode:1989Geo....17.1085W. doi:10.1130/0091-7613(1989)017<1085:CBEFTD>2.3.CO;2. ISSN 0091-7613.
  15. ↑ O'Brien, P.J.; N. Zotov; R. Law; M.A. Khan; M.Q. Jan (2001). "Coesite in Himalayan eclogite and implications for models of India-Asia collision". Geology. 29 (5): 435–438. Bibcode:2001Geo....29..435O. doi:10.1130/0091-7613(2001)029<0435:CIHEAI>2.0.CO;2.
  16. ↑ Joseph Gonzalez; Suzanne Baldwin; Jay B Thomas; William O Nachlas; Paul G Fitzgerald (2019). "First Discovery of Coesite in the Appalachians: Characterization of Prograde Metamorphism in a Taconic Metapelite". AGU Fall Meeting. 2019: V51B–03. Bibcode:2019AGUFM.V51B..03G.
  17. ↑ Joseph Gonzalez; Suzanne Baldwin; Jay B Thomas; William O Nachlas; Paul G Fitzgerald (2020). "Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen". Geology. 48 (10): 947–951. Bibcode:2020Geo....48..947G. doi:10.1130/G47507.1. S2CID 224854495.
  18. ↑ Levien L.; Prewitt C.T. (1981). "High-pressure crystal structure and compressibility of coesite" (PDF). American Mineralogist. 66: 324–333. Archived from the original (PDF) on 2016-06-04. Retrieved 2009-12-15.
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